1. Field of the Invention
The present invention relates to a data track accessing apparatus which accesses a desired track on a record carrier incorporating a number of tracks.
2. Description of the Prior Art
Conventionally, there is an optical data reproduction apparatus which, as a data storage apparatus, focusses and radiates a light beam emitted from a light source, such as a semiconductor laser, on a disc-shape record carrier rotating at a constant rotation speed.
A number of extremely fine tracks each having a 0.6 micron width and 1.6 micron pitch are spirally disposed or are concentrically disposed on the record carrier. For reproducing signals recorded on the record carrier, tracking control is executed to position the light beam on a track, and a photodetector detects the light beam reflected from the record carrier. When executing the tracking control operation to correctly position the light beam on a track, a track-deviation signal is also detected from the light beam reflected from the record carrier. Tracking control is effected by feeding the track-deviation signal back to a tracking actuator which moves the light beam on the record carrier in a direction generally perpendicular to the track direction.
A number of tracks are formed on the record carrier. The track access function is essential for the system to correctly position the light beam on a desired track.
Track access is executed by initially locking the tracking control system, and then, by driving the tracking actuator toward the desired track so that the light beam can traverse tracks, the tracking .control operation is reactivated on the objective track. To stably perform access at a very fast speed, speed control is executed during the access operation so that the light beam can move at a predetermined speed. Data of the moving speed and position of the light beam is necessary for executing the speed control. The moving speed of the light beam is detected from the period of the track-devation signal generated while the light beam moves across the tracks. The position of the light beam is detected by detecting a track-traverse signal designating that the light beam has transversed the center of each track from each track-deviation signal, followed by counting the number of the track-traverse signals from the access starting track. The track-deviation signal contains external disturbance components such as disc damage, influence of the address signal, or various noise components. To precisely detect the speed and position of the light beam, these external disturbance components must be removed. The frequency of the external disturbance components is variable by the moving speed of the light beam. The faster the speed of the light beam, the higher the frequency of the external disturbance components.
The maximum moving speed of the light beam during the access operation is about 1 meter per second, whereas the minimum speed of the light beam just before arriving at the objective track is about 5 mm per second. Accordingly, the frequency of the track-deviation signal is variable throughout an extensive range between 300 Hz and 600 KHz, and thus, it is quite difficult for any conventional circuit having a single characteristic to fully remove those external disturbance components.
To solve this problem, as described in the U.S. Pat. No. 4,096,534, there is such a system which removes external disturbance components by switching characteristics of an analog filter into two stages according to the rotating speed of the magnetic head. However, since this conventional system is based on the hypothesis for determining the frequency of the track-deviation signal to be a maximum of 7 KHz, it is necessary for the system to perform multi-stage switching of characteristics of the analog filter to fully remove external disturbance components from the track-deviation signal having a frequency which is variable in a vast range from 300 Hz to a maximum of 600 KHz. This in turn results in an increased number of capacitors, transistors, and other circuit elements, thus unavoidably expanding the size of circuits. Furthermore, in order to remove noise spike components, it is necessary for the conventional system to sharply reduce the characteristic of the analog filter beyond the cutoff frequency, thus eventually resulting in the complicated circuit structure and an increased number of circuit elements.